2018 ASHS Annual Conference

Cornus florida (flowering dogwood) is an understory tree native to eastern hardwood forests of the United States (U.S.) and utilized as a popular ornamental landscape tree. In the mid-1990s, dogwood powdery mildew (Erysiphe pulchra) reached epidemic levels throughout the C. florida native range causing a drastic rise in production costs. Dogwood powdery mildew is an obligate biotrophic fungus of some big-bracted dogwoods (C. kousa, C. florida, and C. nuttallii). The pathogen exhibits with the white mycelium, conidiophores, and conidia on the adaxial surface of leaves. Disease symptoms include stunted growth of trees, necrosis of young leaves, red pigmentation near infected areas, and decreased flower and fruit production. During the late 1990s, both sexual and asexual stages of E. pulchra were regularly observed, but in recent years, chasmothecia have been rarely detected. Utilizing 15 microsatellite loci, we analyzed 174 E. pulchra samples collected from symptomatic C. florida leaves to assess genetic diversity and population structure of the pathogen. After clone correction, which removed 77 E. pulchra individuals, a total of 97 multilocus haplotypes (MLH) were used for further population analyses. Data were grouped and analyzed as either two sub-populations or eight sub-populations based on geographical sampling. Our study indicated low genetic diversity and a lack of population structure of E. pulchra in the eastern U.S. The index of association differed significantly from 0, signifying asexual or clonal reproduction. Our demographic history data indicated a population bottleneck among the sampling locations. These findings strongly suggest that E. pulchra has become almost exclusively clonal since 1995, which lends support to the hypothesis that E. pulchra is an exotic pathogen introduced to North America.